On 03/02/2019 01:23 AM, Yonghong Song wrote: > On 2/28/19 4:19 PM, Daniel Borkmann wrote: >> On 03/01/2019 12:41 AM, Stanislav Fomichev wrote: >>> On 03/01, Daniel Borkmann wrote: >>>> This work adds BPF loader support for global data sections >>>> to libbpf. This allows to write BPF programs in more natural >>>> C-like way by being able to define global variables and const >>>> data. >>>> >>>> Back at LPC 2018 [0] we presented a first prototype which >>>> implemented support for global data sections by extending BPF >>>> syscall where union bpf_attr would get additional memory/size >>>> pair for each section passed during prog load in order to later >>>> add this base address into the ldimm64 instruction along with >>>> the user provided offset when accessing a variable. Consensus >>>> from LPC was that for proper upstream support, it would be >>>> more desirable to use maps instead of bpf_attr extension as >>>> this would allow for introspection of these sections as well >>>> as potential life updates of their content. This work follows >>>> this path by taking the following steps from loader side: >>>> >>>> 1) In bpf_object__elf_collect() step we pick up ".data", >>>> ".rodata", and ".bss" section information. >>>> >>>> 2) If present, in bpf_object__init_global_maps() we create >>>> a map that corresponds to each of the present sections. >>>> Given section size and access properties can differ, a >>>> single entry array map is created with value size that >>>> is corresponding to the ELF section size of .data, .bss >>>> or .rodata. In the latter case, the map is created as >>>> read-only from program side such that verifier rejects >>>> any write attempts into .rodata. In a subsequent step, >>>> for .data and .rodata sections, the section content is >>>> copied into the map through bpf_map_update_elem(). For >>>> .bss this is not necessary since array map is already >>>> zero-initialized by default. >>>> >>>> 3) In bpf_program__collect_reloc() step, we record the >>>> corresponding map, insn index, and relocation type for >>>> the global data. >>>> >>>> 4) And last but not least in the actual relocation step in >>>> bpf_program__relocate(), we mark the ldimm64 instruction >>>> with src_reg = BPF_PSEUDO_MAP_VALUE where in the first >>>> imm field the map's file descriptor is stored as similarly >>>> done as in BPF_PSEUDO_MAP_FD, and in the second imm field >>>> (as ldimm64 is 2-insn wide) we store the access offset >>>> into the section. >>>> >>>> 5) On kernel side, this special marked BPF_PSEUDO_MAP_VALUE >>>> load will then store the actual target address in order >>>> to have a 'map-lookup'-free access. That is, the actual >>>> map value base address + offset. The destination register >>>> in the verifier will then be marked as PTR_TO_MAP_VALUE, >>>> containing the fixed offset as reg->off and backing BPF >>>> map as reg->map_ptr. Meaning, it's treated as any other >>>> normal map value from verification side, only with >>>> efficient, direct value access instead of actual call to >>>> map lookup helper as in the typical case. >>>> >>>> Simple example dump of program using globals vars in each >>>> section: >>>> >>>> # readelf -a test_global_data.o >>>> [...] >>>> [ 6] .bss NOBITS 0000000000000000 00000328 >>>> 0000000000000010 0000000000000000 WA 0 0 8 >>>> [ 7] .data PROGBITS 0000000000000000 00000328 >>>> 0000000000000010 0000000000000000 WA 0 0 8 >>>> [ 8] .rodata PROGBITS 0000000000000000 00000338 >>>> 0000000000000018 0000000000000000 A 0 0 8 >>>> [...] >>>> 95: 0000000000000000 8 OBJECT LOCAL DEFAULT 6 static_bss >>>> 96: 0000000000000008 8 OBJECT LOCAL DEFAULT 6 static_bss2 >>>> 97: 0000000000000000 8 OBJECT LOCAL DEFAULT 7 static_data >>>> 98: 0000000000000008 8 OBJECT LOCAL DEFAULT 7 static_data2 >>>> 99: 0000000000000000 8 OBJECT LOCAL DEFAULT 8 static_rodata >>>> 100: 0000000000000008 8 OBJECT LOCAL DEFAULT 8 static_rodata2 >>>> 101: 0000000000000010 8 OBJECT LOCAL DEFAULT 8 static_rodata3 >>>> [...] >>>> >>>> # bpftool prog >>>> 103: sched_cls name load_static_dat tag 37a8b6822fc39a29 gpl >>>> loaded_at 2019-02-28T02:02:35+0000 uid 0 >>>> xlated 712B jited 426B memlock 4096B map_ids 63,64,65,66 >>>> # bpftool map show id 63 >>>> 63: array name .bss flags 0x0 <-- .bss area, rw >>> Can we use <main prog>.bss/data/rodata names? If we load more than one >>> prog with global data that should make it easier to find which one is which. >> >> Yeah that's fine, we can change it. They could potentially also be shared, >> so <main prog>.bss/data/rodata might be misleading, but <obj>.bss/data/rodata >> could be. > > Note the map_name field only 16 bytes (excluding ending '\0', only 15 > bytes). If <obj> file has a long name like test_verifier.o, you may have > to shorten the <obj> part of the name. Yes, it needs to be ensured that (bss/)data/rodata part is still visible to the user, so <obj> part would need to be truncated accordingly.
